As a focus adjusting method in an imaging device, focus detection by a phase difference system and a contrast system is known. According to the phase difference system, the focus can be adjusted with high-speed, but it is necessary that a sensor to detect a phase difference is separately from an image detection sensor. On the other hand, according to the contrast system, a focus is adjusted by moving a focus position forward and backward and evaluating a signal from an imaging element by use of a function. Since the focus position can be detected by the imaging element, it is not necessary to provide a sensor separately from the image detection sensor, and therefore, the imaging device can be easily downsized. However, there is a demerit that a focus speed may be slow.
Considering this, an image surface phase difference system is proposed, in which a pixel configured to detect a focus by the phase difference system is embedded in an imaging element (for example, refer to PTLs 1 to 3).
For instance, PTL 1 discloses a configuration in which a focus detecting pixel formed by partly shielding the pixel from light is provided at a predetermined position inside the imaging element. According to the method of PTL 1, while accuracy of focus detection is high, a defective pixel is generated because it is difficult to use a signal of the focus detecting pixel to generate an image. Further, high-density arrangement of the focus detecting pixels may contradict defect density.
On the other hand, PTL 2 discloses a configuration in which two divided photoelectric conversion units are included inside one pixel. The accuracy of focus detection in a single pixel is lower than the accuracy of focus detection by partly shielding light, but in the case of utilizing the configuration to generate an image, no defective pixel is generated because it is only to read a combined signal of both photoelectric conversion units.
According to the configuration in PTL 2, the signals from the two photoelectric conversion units inside one pixel are combined when utilized to generate the image, and therefore, the two photoelectric conversion units inside the one pixel are connected to one floating diffusion (FD) unit. Since capacity of the FD unit is optimized for the two photoelectric conversion units, a signal amount becomes reduced and an S/N ratio may be deteriorated in the case of detecting the focus in the case of using only one of the photoelectric conversion units. In contrast, when the capacity of the FD unit is optimized for one of the photoelectric conversion units at the time of detecting the focus, a dynamic range is reduced and the pixel signal corresponding to a received light amount may not be received.
Additionally, according to any one of the pixel configurations of PTLs 1 and 2, the signal may be obtained from only one photoelectric conversion unit inside one pixel at the time of detecting the focus. Considering this, there is a technology in which a memory unit is provided in each of the two photoelectric conversion units inside one pixel (for example, refer to PTL 3).